This invention is related to active matrix Liquid Crystal Displays (AM-LCDs), and specially to a method for making active matrix LCDs based on non-linear diodes and a method of improving the display uniformity of these diode based AM-LCDs by calibrating individual pixels.
Active matrix Liquid Crystal Displays (AM-LCDs) are one of the major type of flat panel displays that can offer high resolution, high contrast; and fast response time suitable for video applications. Even though active matrix LCDs have better display quality than other kinds of passive matrix LCDs, active matrix LCDs are usually more difficult to manufacture and therefore more expansive. There are generally two broad categories of active matrix displays: one category use three-terminal thin film transistors (TFT) as the switching elements and the other category use two-terminal diodes as the switching elements. Typical two-terminal diodes used in active matrix LCDs are thin film diodes (TFD) and metal-insulator-metal (MIM) diodes Since two-terminal diodes are much easier to manufacture than three-terminal transistors, active matrix LCDs based on two-terminal diodes should be cheaper than active matrix LCDs based on three-terminal transistors, especially for large area displays. At present, however, in market place, active matrix LCDs based on two-terminal diodes have not been as successful as active matrix based on three-terminal transistors, because the display quality of LCDs based on two-terminal diodes have not been as good as the display quality of LCDs based on three-terminal transistors. The major reason for the poor display quality of LCDs based on two-terminal diodes is that, with present known driving techniques, display uniformity of LCDs based on two-terminal diodes usually depend on the uniformity of the characteristics of those two-terminal diodes. Because the characteristics of the two-terminal diodes in a LCD are inevitably non-uniform, correspondingly, the display uniformity of LCDs based on two-terminal diodes are usually not good. Different driving methods have been invented, but they have only achieved very limited success. For example, the driving methods described in U.S. Pat. No. 5,159,325 have only partially solved the problem, and these driving methods have also caused other technical problems, such as the burn-in of images, which are addressed in U.S. Pat. No. 5,648,794.
In this document, the applicant present a new method, which uses diodes to perform the switching function for isolating different pixels. With this method, both terminals of the capacitor for each pixel are used in synchronize for charging the capacitor to a desired voltage level. Terminal one of the capacitor is connected to two diodes. This terminal of the capacitor will effectively connect to the ground with low impedance if the two diodes are switched on with a driving current passing though both of them, and effectively connect to the ground with high impedance if no driving current is passing though them. When this terminal of the capacitor is effectively connected to the ground with low impedance, the second terminal of the capacitor will be set to a voltage level by driver electronics, and this voltage is used to charge the capacitor. With this method, the uniformity problem of the LCD matrix can be easily solved by measuring the reference voltage level of the terminal one of the capacitor once it is effectively connected to the ground with low impedance, and the voltage level on terminal two is set to equal to the sum of two voltages: the reference voltage of the terminal one and the desired charging voltage across the capacitor. This new method provides almost perfectly uniform display properties for active matrix LCDs based on two-terminal diodes regardless the inevitable variations of those diodes. In real operation, the measured reference voltages level of the terminal one of all capacitors can be stored in a calibration memory. When the main processor want to store a pixel""s desired light intensity word to a video memory, it will first fetch the reference voltage of the terminal one of that pixel from the calibration memory, then, calculate what voltage level on terminal two will provide the desired voltage level across the capacitor of that pixel, and finally write the compensated voltage level into the video memory.
In this document, the applicant also demonstrate that present disclosed method of improving display uniformity by storing each pixel""s display characteristics can also be applied to other driving methods for LCDs. In general, present disclosed method of improving display uniformity can be performed in three steps. In the first step, the display characteristics of all pixel element are measured, and the measured characteristics of all pixel element are stored in a calibration memory. In the second step, instead of having the main processor store a pixel""s desired light intensity word directly to a video memory, the main processor will send the desired light intensity word to a register of a microprocessor; the microprocessor will then fetch the display characteristics of the pixel element from the calibration memory to a register or registers; the microprocessor will calculate the compensated light intensity in real time based on the desired light intensity and the display characteristics of the pixel element; the microprocessor finally store the compensated light intensity in a video memory. And in the third step, the compensated light intensities in the video memory are used by the driver electronics to drive the display that can achieve error-free images. Either a stand along special microprocessor or the main microprocessor can be used for the calculation.
It is an object of the invention to provide a method that can provide almost perfectly uniform display properties for active matrix LCDs based on two-terminal diodes regardless the inevitable variations of these diodes.
It is an object of the invention to use two serially connected two-terminal non-linear element as the switching element for each pixel, and such switching element is used to change the effective impedance connecting the capacitor of each pixel to a common ground.
It is an object of the invention to measure the display characteristics of each individual pixel element, store these measured display characteristics into a calibration memory, use the stored display characteristics in the calibration memory to calculate the correct driving parameters for each pixel element, store those corrected driving parameters in a video memory, and use the correct driving parameters in the video memory to drive the active matrix LCD.
It is an object of the invention to measure the display characteristics of each individual pixel element, store those measured display characteristics into a calibration memory, use the stored display characteristics in the calibration memory in combination with the uncompensated driving parameters in a video memory to calculate the correct driving parameters for each pixel, and use the correct driving parameters to drive the active matrix LCD.
It is an object of the invention to provide a method that can provide almost perfectly uniform display properties for active matrix LCDs based on two-terminal diodes of modest quality, regardless the inevitable variations of these diodes, even if these diodes have non-negligible leakage current while in the off-state.
Additional advantages and novel features of the invention will be set forth in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following, or may be learned by practice of the invention. The objects and advantages of the invention maybe realized and attained by means of the instrumentality and combinations particularly pointed out,in the appended claims.
To achieve the foregoing and other objects and in accordance with the present invention, as described and broadly claimed herein, for each pixel, two non-linear elements are provided to connected to terminal one of the capacitor for that pixel; a driving method is provided to switch the impedance of that terminal to the ground between a high value and a low value; a method is provided to measure the reference voltage of terminal one when it is connected to the ground with low impedance; a calibration memory is provided to store the measured reference voltages of all pixels; a microprocessor is provided to use the stored reference voltages in the calibration memory to calculate the correct driving voltage for each pixel; a method is provided to charge the capacitor to the target voltage by setting the terminal two of the capacitor to the correct driving voltage which is already compensated for the variations among those non-linear element. For non-linear element based on diodes of modest quality, a third non-linear element is provided to isolate the terminal two of the capacitor when the voltage on the capacitor need to be maintained.
For any kinds of diode-based AM-LCDs in general, to achieve the foregoing and other objects and in accordance with the present invention, as described and broadly claimed herein, a method is provided to measure the display characteristics of every pixel element in the display; a calibration memory is provided to store the measured display characteristic of every pixel element in the display, a microprocessor is provided to use the stored display characteristics of each pixel element in the calibration memory to calculate the correct driving parameters for the corresponding pixel element, and finally driver electronics are provided to use the correct driving parameters to drive the active matrix display. A diode-based active matrix LCD driven by driver electronics using the correct driving parameters will provide images free of intensity distortions caused by each diode""s property variations.